Gaskets have been made from a variety of structural materials, including polymers such as ethylene/vinyl acetate (EVA) and polyvinyl chloride (PVC). For example, U.S. Pat No. 4,984,703 (Burzynski) discloses plastic closures which have a sealing liner comprising a blend of ethylene/vinyl acetate and a thermoplastic elastomeric composition.
Depending on the use environment, gaskets can have varying degrees of properties. For example, in corrosive service conditions, the gasket must be impervious to the material in question, but still resilient enough to form a seal. Gaskets used in the food and beverage area have similar requirements, but cannot contaminate the foodstuff. For example, when a gasket is used as a bottle cap closure liner and the closure is applied and removed (and/or resealed), it is desireable for the gasket to retain its integrity and not shred or tear (known in the industry as "stringing" or "scuffing") such that pieces of it contaminate the foodstuff. Further, the gasket or closure liner should not deform such that it loses its seal integrity. Depending upon the type of food and/or liquid contents, the filling temperature might be lower or higher than room temperature, thus placing even greater demands on the gasket.
Various attempts to solve these challenges usually involve the use of oil additives or elastomer additives.
For example, U.S. Pat. No. 5,137,164 (Bayer), the disclosure of which is incorporated herein by reference, discloses a method of lining a plastic closure with a thermoplastic. The thermoplastic is a non-cross linked curable, vinyl chloride copolymer composition which has been plasticized with an epoxidized oil, an organic diglycidyl ether and a curing agent for the ether.
U.S. Pat. No. 4,807,772 (Schloss) and U.S. Pat. No. 4,846,362 (Schloss), the disclosure of each of which is incorporated herein by reference, disclose polypropylene and polyethylene closures, respectively, each having removable liners made from a blend of polyethylene and a thermoplastic, elastomeric copolymer (such as a block copolymer of styrene and butadiene). The blends are said to generally include 20-50 weight percent oil.
U.S. Pat. No. 4,872,573 (Johnson et al.), the disclosure of which is incorporated herein by reference, discloses barrier layers for closures selected from the group consisting of ethylene/vinyl alcohol copolymers and polyvinylidene chloride, especially for retarding oxygen containing gases migration.
U.S. Pat. No. 5,000,992 (Kelch), the disclosure of which is incorporated herein by reference, discloses a plastic container closure made from a coextruded multilayer foamed film. The film has at least one solid layer of a polyethylene blend and at least one foamed layer of a second polyethylene blend. The polyethylene blends can be blends of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE). This type of a liner is co-extruded using a blown film or cast film process, unlike the process disclosed and claimed herein. The polymer blend (foamed & unfoamed layers) is used to laminate to other materials such as polyester or metallic films.
U.S. Pat. No. 3,786,954 (Shull), the disclosure of which is incorporated herein by reference, discloses laminated gaskets comprising a combination of a thick foamed polyethylene sheet material and a thin air and moisture impervious SARAN.TM. (trademark of and made by The Dow Chemical Company) layer adhered to the polyethylene by a low density polyethylene (LDPE) bond.
U.S. Pat. No. 5,104,710 (Knight), the disclosure of which is incorporated herein by reference, discloses improvement of gasket adhesion through use of propylene adhesion promoters. Knight also discloses a linear low density polyethylene (LLDPE) as a comparison example and shows that it has insufficient bond temperature of 200.degree. C.
U.S. Pat. No. 4,529,740 (Trainor), the disclosure of which is incorporated herein by reference, discloses foamable structures made from elastomers such as styrene-butadiene block copolymers, a small amount of a salt of a sulfonated styrene polymer, and a blowing agent.
U.S. Pat. No. 4,744,478 (Hahn), the disclosure of which is incorporated herein by reference, discloses a molded closure comprising at least one substantially unfoamed polymer layer and an integrally molded foamed layer of the same polymer. The polymer can be olefins, styrenes, polyesters, polycarbonates, or other suitable engineering resins. A preferred polymer is a copolymer of propylene and EDPM rubber.
Polyvinyl chloride (PVC) polymers have also been used extensively as food closure gaskets, but these are increasingly coming under environmental pressures. Other polymers have also been used for their softness qualities, such as ethylene/methacrylic acid or ethylene/acrylic acid copolymers, but these often times contribute negatively to taste and odor problems, since the polymeric gasket comes in contact with the food and certain polymeric constituents leach into the food.
Higher density polyethylene (HDPE) also has been disclosed as useful for forming gaskets, since the higher density polyethylene has relatively good taste and odor properties, but has not been commercially successful to date, because the polymer is too "hard" and because by adding oil to reduce the hardness, the extractables increase, thus negating regulatory requirements for food contact. In addition, while heterogeneous linear low density polyethylene (LLDPE) has better softness properties than HDPE, this LLDPE does not adhere well to certain plastic closures (e.g., polypropylene, which is often used as a closure material, as described in U.S. Pat. No. 4,807,772) resulting in a loose polyethylene gasket. Further, this heterogeneous LLDPE, depending on the polymer's density, can also adversely affect taste and odor.
While there have been many different solutions to the problems of gasket materials, most of these incorporated additives causing other problems. We have now discovered olefin polymers and olefin polymer compositions useful in making gasket materials, without additives and, in the case of foodstuff, without adversely contributing to the taste and/or odor of the product.